1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a facsimile, and a copying machine.
2. Description of the Related Art
Conventionally, an image forming apparatus including a plurality of image forming units that form images of a plurality of colors including black onto corresponding image carriers has been known, as disclosed in Japanese Patent Application Laid-open No. 2006-201743, for example.
The image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-201743 includes a direct transfer position where a black image formed in a black image forming unit is directly transferred onto a recording medium, and a secondary transfer position where images of the other colors primarily transferred onto an intermediate transfer belt from the image forming units of the other colors are secondarily transferred onto the recording medium. The secondary transfer position is located upstream of the direct transfer position in a recording medium conveying direction. The intermediate transfer belt is rotatably stretched across a plurality of roller members, and a driving roller that is one of the roller members drives the intermediate transfer belt in rotation. A recording medium carriage belt for carrying and conveying the recording medium along a path passing through the direct transfer position and the secondary transfer position is rotatably stretched across a plurality of roller members including a recording medium carriage belt driving roller. In the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-201743, the recording medium carriage belt carries the recording medium through the secondary transfer position and the direct transfer position, so that the black image is superimposed at the direct transfer position over the images of the other colors that have been transferred onto the recording medium at the secondary transfer position so as to form a full color image on the recording medium. Furthermore, by conveying the recording medium in a manner carried on the recording medium carriage belt, fluctuations in a recording medium conveying path between the secondary transfer position and the direct transfer position can be suppressed, and the recording medium can be stably conveyed between the secondary transfer position and the direct transfer position.
In the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-201743, speed fluctuations, which are in the rotation cycle of the recording medium carriage belt driving roller due to factors such as decentering or fluctuating load of the recording medium carriage belt driving roller, cause the rotation speed of the recording medium carriage belt to fluctuate cyclically. In addition, the conveying speed of the recording medium, which is conveyed in the manner carried on the recording medium carriage belt, also fluctuates following the rotation speed of the recording medium carriage belt. Therefore, the conveying speed of the recording medium carried and conveyed on the recording medium carriage belt also fluctuates cyclically, in the same manner as the speed of the recording medium carriage belt. If such speed fluctuations occur in the conveying speed of the recording medium, and if the phase of the fluctuations of the conveying speed of the recording medium is different at the secondary transfer position and at the direct transfer position, the cyclical speed fluctuations of the recording medium carriage belt cause a positional deviation between the images of the other colors and the black image transferred at the secondary transfer position and the direct transfer position, respectively, onto the recording medium.
Furthermore, even if the secondary transfer position is located downstream of the direct transfer position in the recording medium conveying direction, the cyclical speed fluctuations of the recording medium carriage belt cause a positional deviation between the black image and the images of the other colors transferred at the direct transfer position and the secondary transfer position, respectively, onto the recording medium.
To address this issue, the applicant of the present application proposed an image forming apparatus disclosed in Japanese Patent Application No. 2009-133931 (hereinafter, referred to as a “previous application”).
The image forming apparatus disclosed in the previous application includes a direct transfer position where a black image formed in a black image forming unit is directly transferred onto a recording medium, and a secondary transfer position where images of the other colors, primarily transferred from image forming units of the other colors onto an intermediate transfer belt, are secondarily transferred onto the recording medium. The secondary transfer position is located upstream or downstream of the direct transfer position in the recording medium conveying direction. The intermediate transfer belt is rotatably stretched across a plurality of roller members including an intermediate transfer belt driving roller. A recording medium carriage belt for carrying and conveying the recording medium along a path passing through the direct transfer position and the secondary transfer position is rotatably stretched across a plurality of roller members including a recording medium carriage belt driving roller. The interval between the direct transfer position and the secondary transfer position is set to a positive integral multiple of the circumferential length of the recording medium carriage belt driving roller.
In this manner, the phases of the cyclic speed fluctuations of the recording medium carriage belt at the rotation cycle of the recording medium carriage belt driving roller can be synchronized at the direct transfer position and the secondary transfer position; and the phases of the cyclic speed fluctuations of the recording medium, conveyed in the manner carried on the recording medium carriage belt, can be synchronized at the direct transfer position and the secondary transfer position. Therefore, the impact of the cyclic speed fluctuations of the recording medium carriage belt on the conveying speed of the recording medium can be cancelled out between the direct transfer position and the secondary transfer position. Thus, the positional deviation occurs in the images transferred onto the recording medium at the direct transfer position and the secondary transfer position because of the cyclic speed fluctuations of the recording medium carriage belt can be suppressed.
However, a limitation is imposed on the image forming apparatus disclosed in the previous application, because the interval between the direct transfer position and the secondary transfer position must be a positive integral multiple of the circumferential length of the recording medium carriage belt driving roller. The limitation makes it difficult to reduce the interval between the direct transfer position and the secondary transfer position and to reduce the size of the image forming apparatus.